RFC 5062

Security Attacks Found Against the Stream Control Transmission Protocol (SCTP) and Current Countermeasures

Network Working Group R. Stewart
Request for Comments: 5062 Cisco Systems, Inc.
Category: Informational M. Tuexen
Muenster Univ. of Applied Sciences
G. Camarillo
Ericsson
September 2007 Security Attacks Found Against
the Stream Control Transmission Protocol (SCTP)
and Current Countermeasures
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Abstract
This document describes certain security threats to SCTP. It also
describes ways to mitigate these threats, in particular by using
techniques from the SCTP Specification Errata and Issues memo (RFC
4460). These techniques are included in RFC 4960, which obsoletes
RFC 2960. It is hoped that this information will provide some useful
background information for many of the newest requirements spelled
out in the SCTP Specification Errata and Issues and included in RFC
4960.

2.1. Attack Details
+----------+ +----------+ +----------+
| Evil | | Server | | Client |
| IP-A=+------------+ +-----------+=IP-C & D |
| Attacker | | | | Victim |
+----------+ +----------+ +----------+
Figure 1: Camping
Consider the scenario illustrated in Figure 1. The attacker
legitimately holds IP-A and wishes to prevent the 'Client-Victim'
from communicating with the 'Server'. Note also that the client is
multi-homed. The attacker first guesses the port number our client
will use in its association attempt. It then uses this port and sets
up an association with the server listing not only IP-A but also IP-C
in its initial INIT chunk. The server will respond and set up the
association, noting that the attacker is multi-homed and holds both
IP-A and IP-C.
Next, the victim sends in an INIT message listing its two valid
addresses, IP-C and IP-D. In response, it will receive an ABORT
message with possibly an error code indicating that a new address was
added in its attempt to set up an existing association (a restart
with new addresses). At this point, 'Client-Victim' is now prevented
from setting up an association with the server until the server
realizes that the attacker does not hold the address IP-C at some
future point by using a HEARTBEAT based mechanism. See the
mitigation option subsection of this section.
2.2. Analysis
This particular attack was discussed in detail on the SCTP
implementors list in March of 2003. Out of that discussion, changes
were made in the BSD implementation that are now present in
[RFC4960]. In close examination, this attack depends on a number of
specific things to occur.
1) The attacker must set up the association before the victim and
must correctly guess the port number that the victim will use. If
the victim uses any other port number the attack will fail.

2) SCTP's existing HEARTBEAT mechanism as defined already in
[RFC2960] will eventually catch this situation and abort the evil
attacker's association. This may take several seconds based on
default HEARTBEAT timers but the attacker himself will lose any
association.
3) If the victim is either not multi-homed, or the address set that
it uses is completely camped upon by the attacker (in our example
if the attacker had included IP-D in its INIT as well), then the
client's INIT message would initiate an association between the
client and the server while destroying the association between the
attacker and the server. From the servers' perspective, this is a
restart of the association.
2.3. Mitigation Option
[RFC4960] adds a new set of requirements to better counter this
attack. In particular, the HEARTBEAT mechanism was modified so that
addresses unknown to an endpoint (i.e., presented in an INIT with no
pre-knowledge given by the application) enter a new state called
"UNCONFIRMED". During the time that any address is UNCONFIRMED and
yet considered available, heartbeating will be done on those
UNCONFIRMED addresses at an accelerated rate. This will lessen the
time that an attacker can "camp" on an address. In particular, the
rate of heartbeats to UNCONFIRMED addresses is done every RTO. Along
with this expanded rate of heartbeating, a new 64-bit random nonce is
required to be inside HEARTBEATs to UNCONFIRMED addresses. In the
HEARTBEAT-ACK, the random nonce must match the value sent in the
HEARTBEAT before an address can leave the UNCONFIRMED state. This
will prevent an attacker from generating false HEARTBEAT-ACKs with
the victim's source address(es). In addition, clients that do not
need to use a specific port number should choose their port numbers
on a random basis. This makes it hard for an attacker to guess that
number.
3. Association Hijacking 1
Association hijacking is the ability of some other user to assume the
session created by another endpoint. In cases of a true man-in-the-
middle, only a strong end-to-end security model can prevent this.
However, with the addition of the SCTP extension specified in
[RFC5061], an endpoint that is NOT a man-in-the-middle may be able to
assume another endpoint's association.

3.1. Attack Details
The attack is made possible by any mechanism that lets an endpoint
acquire some other IP address that was recently in use by an SCTP
endpoint. For example, DHCP may be used in a mobile network with
short IP address lifetimes to reassign IP addresses to migrant hosts.
IP-A DHCP-Server's Peer-Server
|
|
1 |-DHCP-Rel(IP-A)---->|
2 |------ASCONF(ADD-IP(IP-B), DEL-IP(IP-A)---->XXlost
time
|
|-DHCP-new-net------>|
3 |<---Assign (IP-A)
|
4 |<------------Tag:X-DATA()------------------
|
|-------------INIT()------------------------>
5 |<------------INIT-ACK()---------------------
|
6 |----ASCONF(ADD-IP(IP-Z),DEL-IP(IP-A))------>
Figure 2: Association Hijack via DHCP
At point 1, our valid client releases the IP address IP-A. It
presumably acquires a new address (IP-B) and sends an ASCONF to ADD
the new address and delete to old address at point 2, but this packet
is lost. Thus, our peer (Peer-Server) has no idea that the former
peer is no longer at IP-A. Now at point 3, a new "evil" peer obtains
an address via DHCP and it happens to get the re-assigned address
IP-A. Our Peer-Server sends a chunk of DATA at point 4. This
reveals to the new owner of IP-A that the former owner of IP-A had an
association with Peer-Server. So at point 5, the new owner of IP-A
sends an INIT. The INIT-ACK is sent back and inside it is a COOKIE.
The cookie would of course hold tie-tags, which would list both sets
of tags that could then be used at point 6 to add in any other IP
addresses that the owner of IP-A holds and thus acquire the
association.
It should be noted that this attack is possible in general whenever
the attacker is able to send packets with source address IP-A and
receive packets with destination address IP-A.

3.2. Analysis
This attack depends on a number of events:
1) Both endpoints must support the SCTP extension specified in
[RFC5061].
2) One of the endpoints must be using the SCTP extension for mobility
specified in [RFC5061].
3) The IP address must be acquired in such a way as to make the
endpoint the owner of that IP address as far as the network is
concerned.
4) The true peer must not receive the ASCONF packet that deletes IP-A
and adds its new address to the peer before the new "evil" peer
gets control of the association.
5) The new "evil" peer must have an alternate address, aside from the
IP-A that it can add to the association, so it can delete IP-A,
preventing the real peer from re-acquiring the association when it
finally retransmits the ASCONF (from step 2).
3.3. Mitigation Option
[RFC4960] adds a new counter measure to this threat. It is now
required that Tie-Tags in the State-Cookie parameter not be the
actual tags. Instead, a new pair of two 32-bit nonces must be used
to represent the real tags within the association. This prevents the
attacker from acquiring the real tags and thus prevents this attack.
Furthermore, the use of the SCTP extension specified in [RFC5061]
requires the use of the authentication mechanism defined in
[RFC4895]. This requires the attacker to be able to capture the
traffic during the association setup. If in addition an endpoint-
pair shared key is used, capturing or intercepting these setup
messages does not enable the attacker to hijack the association.
4. Association Hijacking 2
Association hijacking is the ability of some other user to assume the
session created by another endpoint. In cases where an attacker can
send packets using the victims IP-address as a source address and can
receive packets with the victims' address as a destination address,
the attacker can easily restart the association. If the peer does
not pay attention to the restart notification, the attacker has taken
over the association.

4.1. Attack Details
Assume that an endpoint E1 having an IP-address A has an SCTP
association with endpoint E2. After the attacker is able to receive
packets to destination address A and send packets with source address
A, the attacker can perform a full four-way handshake using the IP-
addresses and port numbers from the received packet. E2 will
consider this a restart of the association. If and only if the SCTP
user of E2 does not process the restart notification, the user will
not recognize that the association just restarted. From this
perspective, the association has been hijacked.
4.2. Analysis
This attack depends on a number of circumstances:
1) The IP address must be acquired in such a way as to make the evil
endpoint the owner of that IP address as far as the network or
local LAN is concerned.
2) The attacker must receive a packet belonging to the association or
connection.
3) The other endpoint's user does not pay attention to restart
notifications.
4.3. Mitigation Option
It is important to note that this attack is not based on a weakness
of the protocol, but on the ignorance of the upper layer. This
attack is not possible if the upper layer processes the restart
notifications provided by SCTP as described in section 10 of
[RFC2960] or [RFC4960]. Note that other IP protocols may also be
affected by this attack.
5. Bombing Attack (Amplification) 1
The bombing attack is a method to get a server to amplify packets to
an innocent victim.
5.1. Attack Details
This attack is performed by setting up an association with a peer and
listing the victims IP address in the INIT's list of addresses.
After the association is setup, the attacker makes a request for a
large data transfer. After making the request, the attacker does not
acknowledge data sent to it. This then causes the server to re-
transmit the data to the alternate address, i.e., that of the victim.

After waiting an appropriate time period, the attacker acknowledges
the data for the victim. At some point, the attackers address is
considered unreachable since only data sent to the victims address is
acknowledged. At this point, the attacker can send strategic
acknowledgments so that the server continues to send data to the
victim.
Alternatively, instead of stopping the sending of SACKs to enforce a
path failover, the attacker can use the ADD-IP extension to add the
address of the victim and make that address the primary path.
5.2. Analysis
This attack depends on a number of circumstances:
1) The victim must NOT support SCTP, otherwise it would respond with
an "out of the blue" (OOTB) abort.
2) The attacker must time its sending of acknowledgments correctly in
order to get its address into the failed state and the victim's
address as the only valid alternative.
3) The attacker must guess TSN values that are accepted by the
receiver once the bombing begins since it must acknowledge packets
it is no longer seeing.
5.3. Mitigation Option
[RFC4960] makes two changes to prevent this attack. First, it
details proper handling of ICMP messages. With SCTP, the ICMP
messages provide valuable clues to the SCTP stack that can be
verified with the tags for authenticity. Proper handling of an ICMP
protocol unreachable (or equivalent) would cause the association
setup by the attacker to be immediately failed upon the first
retransmission to the victim's address.
The second change made in [RFC4960] is the requirement that no
address that is not CONFIRMED is allowed to have DATA chunks sent to
it. This prevents the switch-over to the alternate address from
occurring, even when ICMP messages are lost in the network and
prevents any DATA chunks from being sent to any other destination
other then the attacker itself. This also prevents the alternative
way of using ADD-IP to add the new address and make it the primary
address.

An SCTP implementation should abort the association if it receives a
SACK acknowledging a TSN that has not been sent. This makes TSN
guessing for the attacker quite hard because if the attacker
acknowledges one TSN too fast, the association will be aborted.
6. Bombing Attack (Amplification) 2
This attack allows an attacker to use an arbitrary SCTP endpoint to
send multiple packets to a victim in response to one packet.
6.1. Attack Details
The attacker sends an INIT listing multiple IP addresses of the
victim in the INIT's list of addresses to an arbitrary endpoint.
Optionally, it requests a long cookie lifetime. Upon reception of
the INIT-ACK, it stores the cookie and sends it back to the other
endpoint. When the other endpoint receives the COOKIE, it will send
back a COOKIE-ACK to the attacker and up to HB.Max.Burst HEARTBEATS
to the victim's address(es) (to confirm these addresses). The victim
responds with ABORTs or ICMP messages resulting in the removal of the
TCB at the other endpoint. The attacker can now resend the stored
cookie as long as it is valid, and this will again result in up to
HB.Max.Burst HEARTBEATs sent to the victim('s).
6.2. Analysis
The multiplication factor is limited by the number of addresses of
the victim and of the endpoint HB.Max.Burst. Also, the shorter the
cookie lifetime, the earlier the attacker has to go through the
initial stage of sending an INIT instead of just sending the COOKIE.
It should also be noted that the attack is more effective if large
HEARTBEATs are used for path confirmation.
6.3. Mitigation Option
To limit the effectiveness of this attack, the new parameter
HB.Max.Burst was introduced in [RFC4960] and an endpoint should:
1) not allow very large cookie lifetimes, even if they are requested.
2) not use larger HB.Max.Burst parameter values than recommended.
Note that an endpoint may decide to send only one Heartbeat per
RTT instead of the maximum (i.e., HB.Max.Burst). An endpoint that
chooses this approach will however slow down detection of
endpoints camping on valid addresses.
3) not use large HEARTBEATs for path confirmation.

7. Association Redirection
This attack allows an attacker to wrongly set up an association to a
different endpoint.
7.1. Attack Details
The attacker sends an INIT sourced from port 'X' and directed towards
port 'Y'. When the INIT-ACK is returned, the attacker sends the
COOKIE-ECHO chunk and either places a different destination or source
port in the SCTP common header, i.e., X+1 or Y+1. This possibly sets
up the association using the modified port numbers.
7.2. Analysis
This attack depends on the failure of an SCTP implementation to store
and verify the ports within the COOKIE structure.
7.3. Mitigation Option
This attack is easily defeated by an implementation including the
ports of both the source and destination within the COOKIE. If the
source and destination ports do not match those within the COOKIE
chunk when the COOKIE is returned, the SCTP implementation silently
discards the invalid COOKIE.
8. Bombing Attack (Amplification) 3
This attack allows an attacker to use an SCTP endpoint to send a
large number of packets in response to one packet.
8.1. Attack Details
The attacker sends a packet to an SCTP endpoint, which requires the
sending of multiple chunks. If the SCTP endpoint does not support
bundling on the sending side, it might send each chunk per packet.
These packets can either be sent to a victim by using the victim's
address as the sources address, or it can be considered an attack
against the network. Since the chunks, which need to be sent in
response to the received packet, may not fit into one packet, an
endpoint supporting bundling on the sending side might send multiple
packets.
Examples of these packets are packets containing a lot of unknown
chunks that require an ERROR chunk to be sent, known chunks that
initiate the sending of ERROR chunks, packets containing a lot of
HEARTBEAT chunks, and so on.

8.2. Analysis
This attack depends on the fact that the SCTP endpoint does not
support bundling on the sending side or provides a bad implementation
of bundling on the sending side.
8.3. Mitigation Option
First of all, path verification must happen before sending chunks
other than HEARTBEATs for path verification. This ensures that the
above attack cannot be used against other hosts. To avoid the
attack, an SCTP endpoint should implement bundling on the sending
side and should not send multiple packets in response. If the SCTP
endpoint does not support bundling on the sending side, it should not
send in general more than one packet in response to a received one.
The details of the required handling are described in [RFC4960].
9. Bombing Attack (Amplification) 4
This attack allows an attacker to use an SCTP server to send a larger
packet to a victim than it sent to the SCTP server.
9.1. Attack Details
The attacker sends packets using the victim's address as the source
address containing an INIT chunk to an SCTP Server. The server then
sends a packet containing an INIT-ACK chunk to the victim, which is
most likely larger than the packet containing the INIT.
9.2. Analysis
This attack is a byte and not a packet amplification attack and,
without protocol changes, is hard to avoid. A possible method to
avoid this attack would be the usage the PAD parameter defined in
[RFC4820].
9.3. Mitigation Option
A server should be implemented in a way that the generated INIT-ACK
chunks are as small as possible.
10. Bombing Attack (amplification) 5
This attack allows an attacker to use an SCTP endpoint to send a
large number of packets in response to one packet.

10.1. Attack Details
The attacker sends a packet to an SCTP endpoint, which requires the
sending of multiple chunks. If the MTU towards the attacker is
smaller than the MTU towards the victim, the victim might need to
send more than one packet to send all the chunks. The difference
between the MTUs might be extremely large if the attacker sends
malicious ICMP packets to make use of the path MTU discovery.
10.2. Analysis
This attack depends on the fact that an SCTP implementation might not
limit the number of response packets correctly.
10.3. Mitigation Option
First of all, path verification must happen before sending chunks
other than HEARTBEATs for path verification. This makes sure that
the above attack cannot be used against other hosts. To avoid the
attack, an SCTP endpoint should not send multiple packets in response
to a single packet. The chunks not fitting in this packet should be
dropped.
11. Security Considerations
This document is about security; as such, there are no additional
security considerations.
12. References
12.1. Normative References
[RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
Zhang, L., and V. Paxson, "Stream Control Transmission
Protocol", RFC 2960, October 2000.
[RFC4460] Stewart, R., Arias-Rodriguez, I., Poon, K., Caro, A., and
M. Tuexen, "Stream Control Transmission Protocol (SCTP)
Specification Errata and Issues", RFC 4460, April 2006.
[RFC4820] Tuexen, M., Stewart, R., and P. Lei, "Padding Chunk and
Parameter for the Stream Control Transmission Protocol
(SCTP)", RFC 4820, March 2007.
[RFC4895] Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
"Authenticated Chunks for Stream Control Transmission
Protocol (SCTP)", RFC 4895, August 2007.

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